1. Field of the Invention
The present invention relates to an image processing apparatus and a profile generation method. More specifically, the invention relates to gamut mapping in which color gamut of an input device is converted into a color gamut of an output device.
2. Description of the Related Art
In the so-called color management system (CMS) or the like for realizing a color reproduction technology that does not depend on a device between input-output devices, a gamut mapping technology that absorbs a difference in color reproduction range between the input-output devices is used. Conventionally, as one method of gamut mapping, an algorithm of mapping to a point on an auxiliary line that is drawn to one point fixed on a lightness axis (for example, L*=50, a color represented by this point is hereinafter referred to as a focal color) is known.
Japanese Patent Laid-Open No. 2007-274584 discloses an example of gamut mapping using a focal color. Japanese Patent Laid-Open No. 2007-274584 discloses that, when an input image is mapped to an output color gamut by a colorimetric method, the moving range of a focal color is controlled according to the characteristic of mapping such as mapping performed with the emphasis on chroma, mapping performed with the emphasis on lightness and the like, and that furthermore, the focal color is made to be differentiated according to the lightness of an input value.
FIGS. 1A and 1B are diagrams showing a standard color space as a plane specified by a lightness (L*) axis and a chroma (C*) axis in a certain hue in the standard color space. These diagrams illustrate: the range (an upper limit value L_max and a lower limit value L_min) of a focal color fixed on an L axis set under conditions of chroma emphasis and lightness emphasis respectively, with respect to data Pin of a grid point having a lightness defined in the standard color space; and the results obtained by performing the mapping of the data Pin toward the focus within the respective ranges, with a method disclosed in Japanese Patent Laid-Open No. 2007-274584. The focal color is calculated by formula (1) below that adaptively sets the focal color according to the lightness of the grid point. In this formula, L(S) represents the lightness value of the focal color, L_in represents the lightness value of data on the grid point, Lt represents the highest lightness value in an input space and Lb represents the lowest lightness value in the input space.L(S)=(L_max−L_min)·L_in/(Lt−Lb)+L1_min  (1)
In a conventional method, the focal color (Ls, 0, 0) on an achromatic axis (L axis) moves between L_min and L_max, according to the lightness L_in of the input point. According to formula (1), in the case that the difference between L_min and L_max is small, the focal color moves in a narrow range on the achromatic axis, and mapping with the emphasis on chroma is achieved as shown in FIG. 1A. On the other hand, in the case that the difference between L_min and L_max is large, the focal color moves in a broad range on the achromatic axis, and mapping with the emphasis on lightness is achieved as shown in FIG. 1B. Thus, it is possible to select the characteristic of mapping such as mapping with the emphasis on chroma, mapping with the emphasis on lightness and the like and reflect the requirement of a user on the characteristic of mapping.
However, in the mapping using the focal color as disclosed in Japanese Patent Laid-Open No. 2007-274584, a large change of lightness that is called “lightness jumping” may occur depending on the shape of the color gamut of an output device, and consequently, the impression on a color reproduced in an input device disadvantageously appears to be different from that on a color reproduced in the output device.
FIG. 2 is a diagram illustrating this problem, and shows an example where, in a standard color space 301, colors in a color gamut 300 of an input device is mapped using a focal color Q to a color gamut 302 of a printer that is an output device. As shown in FIG. 2, in the mapping using the focal color Q, colors P1in, P2 in and P3 in defined in the standard color space 301 and having the same lightness in the color gamut 300 of the input device are mapped to colors P1out, P2out and P3out, respectively, in the color gamut of the output device. More specifically, the color P1in in the color gamut 300 of the input device, which also exists within the color gamut 302 of the output device, is mapped to the color as it is. On the other hand, the colors P2in and P3in in the color gamut 300 of the input device, which exist outside the color gamut 302 of the output device, are compressed to be points on the surface of the color gamut 302, which are points on straight lines drawn to the focal color Q from the respective colors, by the mapping, respectively. Then, between the colors P2out and P3out resulting from such compression, a large lightness difference occurs. Specifically, the colors P2out and P3out are respectively mapped to the higher lightness side and to the lower lightness side with respect to the color C having the maximum chroma in the color gamut 302 of the output device, and consequently, even if the colors have the same lightness in the color gamut 300 of the input device, the colors are mapped to colors that highly differ from each other in lightness in the color gamut 302 of the output device. When such large lightness difference occurs, the impression on the color reproduced in the input device appears to be different from that on the color reproduced in the output device.
In Japanese Patent Laid-Open No. 2007-274584, since the mapping described above and using the focal color is performed, the lightness jumping described above may occur depending on the shape of the color gamut of the output device.